Quantifying the seasonal and interannual variability of North American isoprene emissions using satellite observations of the formaldehyde column

[ 1] Quantifying isoprene emissions using satellite observations of the formaldehyde ( HCHO) columns is subject to errors involving the column retrieval and the assumed relationship between HCHO columns and isoprene emissions, taken here from the GEOSCHEM chemical transport model. Here we use a 6-year ( 1996 - 2001) HCHO column data set from the Global Ozone Monitoring Experiment (GOME) satellite instrument to ( 1) quantify these errors, ( 2) evaluate GOME-derived isoprene emissions with in situ flux measurements and a process-based emission inventory ( Model of Emissions of Gases and Aerosols from Nature, MEGAN), and ( 3) investigate the factors driving the seasonal and interannual variability of North American isoprene emissions. The error in the GOME HCHO column retrieval is estimated to be 40%. We use the Master Chemical Mechanism (MCM) to quantify the time-dependent HCHO production from isoprene, alpha-and beta-pinenes, and methylbutenol and show that only emissions of isoprene are detectable by GOME. The time-dependent HCHO yield from isoprene oxidation calculated by MCM is 20 - 30% larger than in GEOS-CHEM. GOME-derived isoprene fluxes track the observed seasonal variation of in situ measurements at a Michigan forest site with a - 30% bias. The seasonal variation of North American isoprene emissions during 2001 inferred from GOME is similar to MEGAN, with GOME emissions typically 25% higher ( lower) at the beginning ( end) of the growing season. GOME and MEGAN both show a maximum over the southeastern United States, but they differ in the precise location. The observed interannual variability of this maximum is 20 - 30%, depending on month. The MEGAN isoprene emission dependence on surface air temperature explains 75% of the month-to-month variability in GOME-derived isoprene emissions over the southeastern United States during May - September 1996 - 2001.